Controlled sensitivity igniter composition and method of producing same



United States Patent Ofiice 3,110,638 Patented Nov. 12, 1963 3,110,638 CONTRGLLED SENSlTlVlTY IGNHTER OMPSL TION AND METHOD 8F PRODUCTNG SAME Maurice F. Murphy, West Hyattsville, and Benizmin F. Larrick, Silver Spring, Md, assignors to the United States of America as represented by the decretary of the Navy N Drawing. Filed July h, 1958, Ser- N 747,548 7 Claims. (U. 149-6) (Granted under Title 35, US. tCcde (1952), sec. 266) cone fluids known as the organopolysiloxanes. The compositions to which this invention relates are known as first fire or igniter compositions. That is, they are the materials present in explosive devices which are ignited by a primer and which in turn, by the heat evolved thereby, ignite the explosive filler to produce the eifects desired.

These first fire or igniter compositions must be invariably ignited by the primer. This means that the composition must have a suificiently low ignition temperature. This ignitability depends on several factors such as the nature of the oxidizable metal powder and the oxidant employed in the composition, the particle size and porosity of the ingredients and the composition of the mixture.

To insure ignition of the composition the particles size should be such that at least 99% of the material passes a US. Standard Sieve No. 325. The metal powder employed should have an average diameter in the vicinity of 3 microns. The nature of the surface of the metals employed is important because powdered metals of the same particle size may vary considerably depending on their method of manufacture.

It has been found that to dependably ignite an explosive filler, the igniter composition should be capable of sufficient output of heat and slag while evolving little or no gas since an igniter composition which evolves gas would build up unwanted pressure in a confinedspace and cause ejection of the slag.

igniter compositions ideally should be such as to remain unaffected by exposure to the atmosphere for prolonged periods. To obtain this object, therefore, it is necessary to employ ingredients which are unaffected by moisture.

The number of constituents of an igniter composition should be kept at 'a minimum and the materials should be available in quantities sufiicient to supply all requirements. Furthermore these igniter compositions must be safe enough to manufacture and load'into explosive devices. This means that the compositions must besufiiciently insensitive to friction, impact, sparks and heat to be handled with the ordinary precautions for handling sensitive pyrotechnic mixtures.

Heretofore, no igniter composition has been developed which has all the desired properties stated above.

igniter compositions which meet many of the requirements set out above have been prepared employing a combination of iron oxide and a powdered titanium or zirconium metal. These igniter compositions have excellent stability in moist atmospheres. They have the desired functioning properties of sensitivity to ignition and development of high temperatures for ignition of 'l hermittype compositions which have ignition temperatures well above 1000 C. These igniter compositions, however, are not satisfactory from the standpoint of safety. This is particularly true of those compositions employing zirconium. Finely divided powders of zirconium and titanium of the size employed in these igniter compositions are extremely sensitive to friction and spark. This is particularly true of powders of these metals prepared by the calcium reduction process. In addition, igniter compositions prepared from powdered titanium and iron oxides have been found unreliable in attempts to ignite them at low temperatures as compared to the ignition of such compositions utilizing zirconium powders.

Attempts have been made to control the friction and spark sensitivity of the compositions employing finely divided zirconium by coating the metallic particles with a wax or other organic materials. Such attempts usually resulted in complete or nearly complete desensitization thus producing an inert or nearly inert material unsuitable for use in igniter composition.

Other attempts to apply films to zirconium powders have employed dissolved solids. The resulting films were found to be loosely bonded. The conditions of application of the films are critical and the resulting films were easily removed by friction during the use of the powders. Also many of these compositions so coated gave off large volumes of gas during the burningof the powder. The resulting compositions have frequently shown either insuflicient change or too great a change in sensitivity and burning rates.

It is, therefore, an object of the present invention to provide igniter compositions employing finely divided powders of zirconium which compositions have controlled sensitivity to friction and sparks.

Another object of the invention is to provide a method of preparing igniter compositions employing finely di- 'vided Zirconium powders which igniter compositions have controlled sensitivity to friction and sparks.

A further object is to provide a new and improved method of controlling the friction and spark sensitivity of powders of finely divided zirconium and of igniter compositions prepared therefrom.

A still further object is to provide zirconium-oxidant igniter compositions having the same particle size and physical structure but possessing different burning rates and sensitivities to friction, spark and flame.

Other objects and the attendant advantages of the invention will become apparent to those skilled in the art as the invention is disclosed in the following detailed description.

The above objects are achieved in accordance with the invention by coating the zirconium powder particles, or the particles of the compositions including such powders, with one or more compounds selected from that class of compounds known as organopolysiloxanes. The sensitivity of the zirconium powder or zirconium powder composition is controlled by varying the thickness of the film of the organopolysiloxane on the surface of the metal particles.

The oxidants employed in the igniter compositions of the invention should have a high degree of purity and be free of gas forming and hygroscopic impurities. In order to obtain good binding qualitiesit has been found that these oxidants should be finely divided in order to form a compact coherent cake upon consolidation under pressure. Iron oxides such as for example Fe O Fe O and FeO may be used in this type of composition but other oxidants known to those skilled in the art as useful in metal-oxidant igniter compositions such as barium chro- Q mate, lead dioxide, cupric oxide and potassium dichromate, may also be employed in these compositions.

Zirconium powders prepared by any suitable process may be employed in the igniter compositions of the invention. However, metallic powders prepared by the method known to those skilled in the art as the calcium reduction method may be considerably more sensitive to ignition than those prepared by other methods. The powders, by whatever process they are prepared, should have a purity as high as possible, preferably over 95%. Metal-oxidant igniter compositions employing aluminum, magnesium and manganese as well as titanium and zirconium may also be desensitized utilizing the process of the invention. However, the preferred composition of the invention employing about 65% zirconium, 25% ferric oxide and 10% diatomaceous earth has been found definitely superior to any of these in reliability to ignition at low temperatures.

The diatomaceous earth is an inert substance added to igniter composition to improve retention of the slag and to increase the mixing homogeneity and strength of the compacted powder.

Igniter compositions to which the invention applies may contain from about 40 to 70 parts by weight of zirconium powder and from about 25 to about 50 parts by weight of oxidant. However, within the above limits the proportions may be varied to produce the desired effects.

The organopolysiloxane fluids which may be employed to control the sensitivity of the igniter compositions in accordance with the invention are those organopolysiloxanes which have suitable viscosity, molecular weight and surface tension. In judging the compound for a particular purpose the permanence of the film to be deposited on the surface of the particles must be considered. For permanence and resistance to aging, the film should be formed of a material having a high boiling point and a high latent heat of vaporization. These properties in general correlate with increasing viscosity and molecular weight.

The extent of sensitivity control appears to depend on the thickness of the film and on its composition. Thus the extent of sensitivity control may be varied by varying (a) the organopolysiloxane or mixture of organopolysiloxanes employed, (b) the concentration of the organopolysiloxane in the liquid medium used to apply it and (c) the method of application of the film.

Thus the effect of the film on the sensitivity of the composition depends on the chemical as well as the physical properties of the organopolysiloxane employed. The effect also varies according to the nature of the alkyl and/ or aryl substituents of the organopolysiloxane.

A siloxane is a silicon compound in which the silicon atoms are connected by oxygen bridges and the remaining valencies of the silicon atoms are saturated with hydrogen or organic radicals. Compounds which consist of chains of -R SiO- groups wherein R is an alkyl or aryl group have received the name siloxane.

The siloxanes may be prepared by the action of alkyl- Mg salts on SiCL; in such a way that the relative quantities of the starting materials used lead to the formation of dialkysilicon dichlorides 2C H MgCl +SiCl (C H SiCl -l- 2MgCl The dialkysilicon dichlorides can be hydrolyzed to the corresponding hydroxy compounds, the silicols, which are unstable and polymerize immediately to poly-dialkysiloxanes:

The polysiloxane fluids which are useful in the present invention are those siloxanes in which the configuration of the molecules is a simple chain rather than a cross linkage since the latter have the characteristics of resins. Siloxanes of the former type which have been found particularly useful in the process of the invention are dimethylpolysiloxane, and methylphenylpolysiloxane although other siloxanes having suitable properties may also be used. The viscosity range of these siloxanes ranges from 0.65 cts. to about a million centistokes at 25 C. Those siloxanes having a viscosity of from about 50 centistokes to about centistokes at 25 C. have been found to give the most satisfactory results when utilized in accordance with the present invention. However, siloxanes having viscosities above and below this range may also be used in the process of the invention.

Where dimethylpolysiloxane and methylphenylpolysiloxane are employed the siloxane may comprise from about 0.5 to about 5.0 percent by weight of the metal powder.

The organopolysiloxane fluid may be applied to the metal powder before it is mixed with the other ingredients of the igniter composition or it may be applied to the powder composition after it has been mixed. In either case the primary object is to coat the metal particles with a coating of an organopolysiloxane fluid having the desired thickness and composition so as to produce the eflects desired.

The organopolysiloxane coatings of the invention may be deposited using suitable solvents completely miscible with the polysiloxane. The preferred solvents are ethyl acetate, and carbon tetrachloride although others such as chloroform, amyl acetate, benzene, kerosene, gasoline, ethylene dichloride, ethyl ether, 2 ethyl hexanol, n-hexyl ether, methyl ethyl ketone, Mineral Seal Oil, BMT naptha, perchloroethylene, Stoddards solvent, trichloroethylene, and xylene may also be employed under certain conditions. Some of the above solvents possess properties which make their use less desirable such as high flammability, low volatility or potential reactivity with metals. Such secondary considerations limit the useful solvents to those which can be safely handled in contact with metal powder and its mixtures and quickly removed after treatment.

Some solvents that are only partially miscible with the polysiloxane employed may be used under some conditions such as acetone, butanol, ethanol, heptadeconol and isopropanol. The secondary considerations which limit the useability of the completely miscible solvents will also limit the use of the partially miscible solvents.

The organopolysiloxane fluid coating or film is applied to the metal powder alone, or to the whole igniter composition, by mixing the powder in an ethyl acetate solution of the siloxane such as in a ball mill or by wetting the powder with a carbon tetrachloride solution of the siloxane. In either case the solvent for the siloxane is subsequently removed from the composition.

The carbon tetrachloride solution method must be used with caution, however, since mixtures of certain metals, such as zirconium, aluminum and magnesium, with carbon tetrachloride are explosive under some conditions of vibration, agitation and confinement. This method should, therefore, be employed under controlled conditions and without agitation of the powder. Adequate desensitization of the powder has been accomplished, however, under these conditions without agitation of the powder.

Once the metal powder has been coated with the siloxane fluid the metal powder may be mixed with the other ingredients to form the igniter composition in any manner desired. The mixing may be accomplished dry or in any suitable liquid medium which will not dissolve the siloX- ane coating applied to the metal powder.

Whatever technique is used to prepare the igniter composition, it should be carefully dried prior to use in order to remove any trace of solvent or other impurities which the latter is above the former.

might form gas or otherwise interfere with the proper operation of the igniter composition.

In order that the invention may be more completely understood the following examples are given. It is understood, however, that-these examples are illustrative only and are not to be considered as limiting the invention in any manner.

The friction sensitivity .test figures given in the examples were obtained through the use of a sliding rod friction tester such as is used at the US. Naval Powder Factory, Indian Head, Maryland. Briefiy described, this tester is composed of rod of standard size and weight which is allowed to slide a standard distance down a runaway inclined at a definite angle to the horizontal so that it strikes a sample of the composition to be tested located on an anvil. The rod is provided with an adjustable end so that the surface of that end is always substantially parallel to the surface of the sample tested. As the angle of the rod to the horizontal increases, the rod creates less friction and more impact on the surface of the sample.

The sensitivity to spark was tested on the spark sensitivity tester used at the U8. Naval Ordnance Laboratory, Silver Spring, Maryland. This tester consists of a stationary electrode and an adjustable electrode placed so that The sample to be tested is placed on the stationary electrode. A predetermined difference in potential is built up between the electrodes and the movable electrode is moved momentarily to a point which is a standard distance from the stationary electrode so that a spark of predetermined energy jumps between the electrodes. The spark energy required to ignite the sample is then recorded.

All tests were conducted on standard size compacts of the powders prepared from the dry powders either before or after coating as described.

EXAMPLE I Substantially pure zirconium powder prepared by the calcium reduction process by charging mixtures of GP. ZrO (-200 mesh) and granulated calcium in measured excess to a steel reduction bomb (approximately 3 /2 in. diameter x 7 in. high), igniting the mixture by placing the bomb in an electric furnace at 900 C., and after cooling, removing the contents and washing the product with water and HCl. The powder which passed a 120 mesh screen was tested in the friction and spark sensitivity testers. In the spark sensitivity tester in 20 attempts 50% of the powders fired at 125 ergs (1 kv.). In the friction sensitivity tester in five attempts 100% ignition was accomplished at an angle of the rod to the horizontal of 45. When the angle of the rod was increased to 55, in five attempts 80% resulted in ignition.

EXAMPLE :II

Fifteen grams of the same type of zirconium powder as in Example I were ball milled at room temperature using /2 inch porcelain balls for /2 hour in a paste of the powder in ethyl acetate containing of a dimethylpolysiloxane having a viscosity of 50 centistokes at 25 C.

and an average molecular weight of 3,800. The ethyl acetate-siloxane solution was then filtered off and the powder dried. In the spark sensitivity tester 20 attempts at 5000 ergs (2.5 kv.) produced no ignition. Twenty attempts at 50,000 ergs (5 kv.) produced ignition in 50% of the tests. In the friction sensitivity tester 25 attempts at an angle of rod of 55 produced no ignition.

EXAMPLE 1 H Three grams of the same type of zirconium powder employed in Example I was mixed at room temperature with 3 ml. of an ethyl acetate solution containing 5% by volume of a =methylphenylpolysiloxane having a viscosity of about 75 centistokes at 25 C. The mixture was then filtered and dried to evaporate the ethyl acetate. I11 the spark sensitivity tester no ignition was produced in 20 attempts at 5000 ergs (5 kv.). Fifty percent ignition was accomplished in 20 attempts at 20,000 ergs (5 kv.).

EXAMPLE 1V Table I 15 Angle of Rod, degrees No. of Results,

Tests ignitions EXAMPLE V The same igniter composition as in Example IV was ball milled in an ethyl acetate siloxane solution as described in Example II. In the friction sensitivity tester the following results were obtained in two series of tests.

Table II Results, ignitions Number of Tests Angle of Rod, degrees Series A Series B Series A and B show the possibility of reproducibility of 40 the process of the invention and are in contrast with those results obtained in Example IV.

EXAMPLE VI Table III Percentage of dimethylpoly- Percentage of dimethylsiloxane dcslloxuue in ethyl acetate solution Results in Spark Sensitivity Tester in Percent of ignitions posited on powder by wt.

0 at 150 ergs 1 kv.

.4 50% at 300 ergs 1 kv. 4 50% at 650 ergs 1 kv. 7 50% at ergs 1 kv. 4 50% at 2,000 ergs at 5 kv.

M mi- In the above example the non-linearity of the effect of concentration was due to failure to strictly control the temperature and time of exposure factors.

EXAMPLE VII Samples of the igniter powder composition of Example IV were placed in a filter funnel and a solution of dimethylpolysiloxane having a viscosity of 50 centistokes at 75 C., average molecular wt. 3800, in carbon tetrachloride Was drawn rapidly through the powder. The powder was then completely dried before testing. The results of this series of tests are as follows:

Table IV Table VI Percent of diamethylpoly- Percent of Results in Spark Sen- Percent of Sensitivity siloxane in sol. by vol. dimethylsisitivity Tester in Pcr- Sensitivity of original Siloxane Sensitivity after after washing loxane deposcent of Ignitions powder (Percent of deposited coating (Percent with CO1; ited by wt. r Ignitions) by wt. of Ignitions) (Percent of Ignitions) 0.0 100% at 150 ergs l kv. 0. 4 50% at 550 ergs 1 kv. 100% at 150 ergs 1 km... 0. 4 50% at 300 ergs 1 100% at 150 0. 7 50% at 900 ergs 1 kv. kv. ergs 1 kv. 1. 1 50% at 1,600 crgs kv. Do 1. 4 50% at 650 ergs 1 D0. 1. 4 50% at 2,600 ergs 5 kv. v. 1. 8 50% at 5,000 ergs 5 kv. 10 Do 2. 7 50% at 425 ergs 1 D0.

V. D0 4. 4 50% at 2,000 ergs 1 D0. EXAMPLE VIII Samples of the Powder f' as m The burning rate of the igniter compositions in which Example IV but using a methylphenylpolysiloxane hav- 15 tbs meta powder is coated with a film or origanopoly fi 'z fi w i i 75 wmlstokes 25 with siloxane fluid will be slowed in proportion to the amount 6 owing of siloxane fluid deposited on the metal powder. The

T able V burning rate of the untreated ignition powder of Ex- P t f ample IV for instance was found to vary from about 90 cream 0 Percent of methylphenylmethylphenyl- Results in Spark sento about 1 1O m11hseCondS fl mch when sofnpacted a polysiloxane by vol. polysiloxane sitivity Tester (Per- 30,000 p.s.1. 111 a hollow cylinder of .203 internal d1- %%i, f cent oflgmtions) ameter and 0.5" outside diameter, aluminum 24 st. This same ignition powder treated as described in Example 8 as 50% at 600 egg 1 IV and loaded in the same manner was found to have a 12 4.0 50% at 625 ergs 1 kv. 25 rate varying between about 300 to about 400 milliseconds;

per inch.

From the foregoing it may be seen that the spark and friction sensitivity of metal-oxidant igniter powder compositions may be varied by varying the thickness of a film of organopolysiloxane fluid deposited on the metal powder particles. The proportion of the siloxane fluid deposited on the surface of the metal powder varies almost proportionately with the concentration of the siioxane fluid in the treating solution. The friction and spark sensitivities of the metal powders and metal-oxidant .igniter compositions prepared therefrom vary as an inverse function of the amount of organopolysiloxane fluid deposited on the surface of the metal powder. The amount of siloxane fluid deposited may vary from about 0.5% up to about 5% by weight of the metal powder.

The desensitized powders prepared in accordance with the invention may have their spark and friction sensitivity restored by treatment of the desensitized metal powder, or the igniter powder prepared therefrom, with solvents for the organopolysiloxane fluid coating. Since the polysiloxane coating is not chemically bonded to the powder, the coating may be removed by washing the desensitized powder with one of the solvents previously described. The ratio of the amount of solvent required to remove the coating to the powder washed is a direct function of the concentration of the polysiloxane deposited.

The coating may be removed completely or in part to increase the sensitivity of the treated powder by any suitable method such as by agitation of the powder with ethyl acetate in a ball mill. The coating may also be removed by passing carbon tetrachloride without agitation through a bed of the powder. Pantially miscible solvents may also be used to remove the siloxane coating under special conditions such as agitation or Washing with large quantities of the solvent.

The following example is given to demonstrate the removability of the desensitizing siloxane coating.

EXAMPLE 1X parison with the sensitivity of the powder before treatment and after desensitization.

From the foregoing it may be seen that there has been provided a method of producing a metal powder for use in igniter compositions and compositions produced therefrom which have a regulated or controlled sensitivity to friction and spark. In addition there has been provided a metal powder for use in igniter compositions having a controlled sensitivity to friction and spark and an igniter composition of controlled sensitivity prepared from this metal powder having greatly improved handling and safety characteristics.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An igniter composition having controlled sensitivity to friction and sparks consisting essentially of a finely divided mixture of from about 40 to about 70 parts by weight of zirconium and from about 25 to about 50 parts by weight of an oxide of iron, said zirconium having a coating of fluid, straight chain organopolysiloxane of the structural unit corresponding to the formula R R SiO Where R is an alkyl radical and R is a member of the group consisting of alkyl and phenyl radicals.

2. An igniter composition having controlled sensitivity to friction and sparks consisting essentially of a finely divided mixture of from about 40 to about 70 parts by weight of zirconium and from about 25 to about 50 parts by weight of an oxide of iron, said mixture having a coating of a fluid, straight chain organopolysiloxane of the structural unit corresponding to the formula R R SiO where R is an alkyl radical and R is a member of the group consisting of alkyl and phenyl radicals.

3. An igniter composition having controlled sensitivity to friction and sparks consisting essentially of a finely divided mixture of about 65 parts by weight of zirconium powder, about 25 parts by weight of ferric oxide powder and about 8.5 parts by weight of diatomaceous earth, said mixture having from about 0.325 part by weight to about 325 parts by weight of a coating of a fluid, straight chain organopolysilox-ane of the structural unit corresponding to the formula R R SiO where R is an alkyl radical and R is a member of the group consisting of alkyl and phenyl radicals.

4. An igniter composition having controlled sensitivity to friction and sparks consisting essentially of a finely divided mixture of about 65 parts by weight of zirconium powder, about 25 parts by weight of ferric oxide powder and about 8.5 parts by weight of diatomaceous earth, said zirconium powder having from about 0.325 part by weight to about 3.25 parts by weight of a coating of a fluid, straight chain organop-olysiloxane oi the structural unit corresponding to the formula R R SiO where R is an alkyl radical and R is a member of the group consisting of alkyl and phenyl radicals.

5. The method of preparing a metal-oxidant igniter composition having controlled sensitivity to friction and sparks =by wetting finely divided zirconium powder with a solution of a fluid, straight chain organopolysiloxane of the structural unit corresponding to the formula R R SiO where R is an alkyl radical and R is a member of the group consisting of alkyl and phenyl radicals in a solvent selected from the .group consisting of ethyl acetate and carbon tetrachloride, removing said solvent from said powder and mixing from about 40 to about 70 parts by weight of the said powder with from about 25 to about 50 parts by weight of a finely divided oxide of iron, and subsequently washing said mixture with a solvent for said organopolysiloxane until said organopolysiloxane constitutes from about 0.5 to about 8.0 parts by weight of said mixture.

6. The method of preparing a metal-oxidant igniter composition having controlled sensitivity to friction and sparks by 'wetting a finely divided mixture of from about 40m about 70 parts by weight of zirconium and from about 25 to about 50 parts by weight of an oxide of iron with a solution of fluid organopolysiloxane of the structural unit corresponding to the formula R R Si Where R is an alkyl radical and R is a member of the group consisting of alkyl and phenyl radicals in a solvent selected from the group consisting of ethyl acetate and carbon tetrachloride, removing said solvent from said mix- 10 ture, and subsequently washing said mixture with a solvent for said organopolysiloxane until said organopolysiloxane constitutes from about 0.5 to about 8.0 parts by weight of said mixture.

7. The method of preparing a metal oxidant igniter composition having controlled sensitivity to friction and spark by coating with a fiuid organopolysiloxane of the structural unit corresponding to the formula R R SiO where R is an alkyl radical and 'R is a member of the group consisting of alkyl and phenyl radicals, a finely divided mixture of from about 50 to about 70 parts by weight of zirconium and from about to about parts by weight of an oxide of iron and subsequently washing said coated mixture with a solvent for said organopolysiloxane until said onganopolysiloxane constitutes from about 0.5 to about 8.0 parts by Weight of said mixture.

References Cited in the file of this patent UNITED STATES PATENTS 2,457,860 Bennett et al. Ian. 4, 1949 2,478,918 Hale Aug. 16, 1949 2,494,920 Warrick Ian. 17, 1950 2,563,305 Britten et al. Aug. 7, 1951 2,864,678 Fink et a1 Dec. 19, 1958 2,986,874 Di Giorgio June 6, 1961 OTHER REFERENCES Kirk and Othrner: Encyclopedia of Chemical Technolog vol. 12, pages 400-403. 

1. AN IGNITER COMPOSITION HAVING CONTROLLED SENSITIVITY TO FRICTION AND SPARKS CONSISTING ESSENTIALLY OF A FINELY DIVIDED MIXTURE OF FROM ABOUT 40 TO ABOUT 70 PARTS BY WEIGHT OF ZIRCONIUM AND FROM ABOUT 25 TO ABOUT 50 PARTS BY WEIGHT OF AN OXIDE OF IRON, SAID ZIRCONIUM HAVING A COATING OF FLUID, STRAIGHT CHAIN ORGANOPOLYSILOXANE OF THE STRUCTURAL UNIT CORRESPONDING TO THE FORMULA R1R2SIO WHERE R1 IS AN ALKYL RADICAL AND R2 IS A MEMBER OF THE GROUP CONSISTING OF ALKYL AND PHENYL RADICALS. 